Rework of bf_decode code to include environment variable configuration

Added the following environmental variables to allow the defaults
to be overriden:

PREFIX - change the filepath prefix (default /data2/latest)
FULLBAND - record entire band instead of inner 50% (default 0)
TRANPOSE - tranpose time frequency blocks from correlator (default 0)

e.g.

 $ PREFIX=/home/kat/data TRANPOSE=1 speadrx -d bf_decode.so

The restriction on a specific beam has also been removed and bf_decode
will now record any beamformer data arriving on the specified port.